The present invention is related to a demodulating circuit for demodulating a reception signal in a wireless receiving apparatus such as a pager and also related to a demodulating method. More specifically, the present invention is directed to a demodulating circuit and a demodulating method employed in a wireless receiving apparatus, in which a modulation signal converted into an intermediate frequency band by the FSK modulation is demodulated to output a demodulation signal.
In demodulating circuits of wireless receiving apparatuses, operating parameters of respective circuit portions are set so as to optimize S/N of signal reception systems. In the case that a modulated multi-level FSK signal is demodulated in, for example, the FLEX system pager (will be referred to as an xe2x80x9cFLEX pagerxe2x80x9d hereinafter), an amplitude value of an output voltage of a detector is set in such a manner that S/N can be optimized in response to a transfer speed of data. Also, a reference voltage value of a data comparator for outputting a digital demodulation signal is set in such a manner that an error judging rate can be minimized. Also, in a low-pass filter provided between the detector and the data comparator, a cutoff frequency is set in response to the data transfer speed in such a manner that S/N can be optimized by removing an unnecessary frequency component. In demodulating circuits employed in mobile communication appliances such as the above-explained FLEX pager, generally speaking, various sorts of setting values are determined under such a condition that electric field strengths are low so as to optimize S/N. As a result, performance of wireless receiving apparatuses may be improved.
As previously explained, in the conventional demodulating circuits of the wireless receiving apparatuses, the various sorts of setting values are determined in such a way that S/N can be optimized under such a condition that the electric field strengths are low. However, there are certain possibilities that when the electric field strengths are high, the above-explained setting values are not equal to the optimum setting values, resulting in lowering of the reception performance.
More specifically, in a pager system where transmission signals are transmitted from a plurality of base stations at the same time, the following difficulties are revealed. That is, in an area where electric field levels of transmission signals (FSK-modulation waves) transmitted from the plural base stations become substantially equal to each other (namely, overlap (equi-field) area), or in an area where a multipath occurs in transmission signals (namely, multipath area), electric field strengths at reception points are largely varied by receiving adverse influences caused by frequency shifts among the plural transmission signals and by phase delays. As a result, bit error rates of demodulation signals would be deteriorated. Under such a condition that the overlap and/or the multipath occur in an area, generally speaking, in such an area, electric field strengths are higher than medium field strengths. Therefore, even when the setting values determined by the above-explained low electric field strengths are used, better reception performance could not be achieved. Also, as reception performance with respect to a frequency shift between a set frequency of a receiving apparatus and a frequency of a transmission signal, a so-called xe2x80x9cband characteristic (i.e., bandwidth characteristic of calling rate) is known in this technical field. It is also revealed that a setting value at which a maximum S/N value can be obtained is not always made coincident with another setting value at which a maximum value of a band characteristic.
As a consequence, the following problem occurs in the conventional demodulating circuit arranged by that S/N is optimized under such a condition that the electric field strength is low. That is, the better reception performance can be obtained under such a low electric field strength. However, the reception performance would be deteriorated in the overlap area and the multipath area existed in the medium electric field strength and the low electric field strength, so that the calling rate would be lowered.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a demodulating circuit of a wireless receiving apparatus capable of performing optimum demodulating operations in various electric field strengths such as a low electric field strength and higher than a medium electric field strength by controlling a detecting means, a filter means, and a comparing means provided in the demodulating circuit of the wireless receiving apparatus such as a pager in response to the electric field strength. Also, the present invention owns an object to provide a demodulating method capable of executing optimum demodulating operations even under such electric field strengths.
To solve the above-described problems, a demodulating circuit of a wireless receiving apparatus according to first aspect of the present invention which demodulates a received modulation signal to output a demodulation signal, comprises: electric field level detecting means for detecting an electric field strength of a reception signal based upon the modulation signal; detecting means for detecting the modulation signal to output a detection signal; filter means for removing a noise component from the reception signal; comparing means for comparing the detection signal with a predetermined reference voltage to output a digital signal in correspondence with the modulation signal; and control means for controlling at least one of an amplitude of the voltage outputted from the detecting means, the reference voltage used in the comparing means, and a cutoff frequency of the filter means in correspondence with the electric field strength detected by the electric field level detecting means.
Also, preferably, the detecting means includes a variable gain amplifier; and the control means controls the amplitude of the output voltage of the detecting means by controlling an amplification factor of the variable amplifier. Also, the control means controls at least one of the output voltage amplitude of the detecting means and the reference voltage used in the comparing means in response to the electric field strength, so that a ratio of the output voltage amplitude of the detecting means to the reference voltage used in the comparing means is controlled to a predetermined value.
Also, preferably, the filter means is arranged by comprising a low-pass filter provided at a pre-stage of the comparing means, and a digital filter provided at a post-stage of the comparing means. Also, the control means selectively controls passing, or non-passing of the digital filter in response to the electric field strength. Alternatively, as recited in claim 6, the control means controls a frequency of a clock signal entered into the digital filter in response to the electric field strength.
Otherwise, the filter means is arranged by a low-pass filter provided at a pre-stage of the comparing means; and the control means controls a cutoff frequency of the low-pass filter in response to the electric field strength.
Also, preferably, the demodulating circuit is further comprised of memory means for storing thereinto control condition information related to a plurality of the output voltage amplitudes, a plurality of the reference voltages, and a plurality of the cutoff frequencies, which are set in correspondence with the electric field strength; and the control means performs the control operation in accordance with both electric field strength information indicative of the electric field strength and the control condition information.
A demodulating method of a wireless receiving apparatus according to second aspect of the present invention for demodulating a received modulation signal to output a demodulation signal, comprising: an electric field level detecting step for detecting an electric field strength of a reception signal based upon the modulation signal; a detecting step for detecting the modulation signal to output a detection signal; a noise removing step for removing a noise component from the reception signal; a digitalizing step for comparing the detection signal with a predetermined reference voltage to output a digital signal in correspondence with the modulation signal; and a control step for controlling at least one of an amplitude of the voltage of the detection signal, the reference voltage, and a cutoff frequency used in the noise removing step in correspondence with the detected electric field strength.
Also, preferably, in the control step, at least one of the output voltage amplitude of the detection signal and the reference voltage is controlled in response to the electric field strength, so that a ratio of the output voltage amplitude to the reference voltage is controlled to a predetermined value. Also, in the control step, when said electric field strength is smaller than a preselected threshold value, the cutoff frequency used in removing of the noise is controlled to be lower than that when said electric field strength is larger than the threshold value.
In the demodulating circuit of the wireless receiving apparatus according to the present invention, the detection signal obtained by detecting the received modulation signal is outputted, the noise component is removed from the detection signal, and then this detection signal is compared with a predetermined reference voltage. As a result, the digital signal corresponding to the modulation signal is outputted from the demodulating circuit. Then, the demodulating circuit judges this digital signal at predetermined timing to produce the digital demodulation signal. At this time, the electric field strength of the reception signal is detected by the modulation signal, and the demodulating circuit controls at least one of the voltage amplitude of the detection signal, the reference voltage, and the cutoff frequency used in removing of the noise component in response to the electric field strength of the detected reception signal. As a consequence, the demodulating circuit is controlled in such a way that the ratio of the voltage amplitude of the detection signal to the reference voltage, which as used to produce the digital signal, can become the optimum setting value, and also the cutoff frequency of the filter means can become the optimum setting value in response to the electric field strength.
In order to control the ratio of the voltage amplitude of the detection signal of the reference voltage, either on or both the output voltage of the detecting means and the reference voltage used in the comparing means are varied. In this case, the amplification factor of the amplifier provided in the detecting means is varied, and both the output from the variable current source and the resistance value of the resistor, which may define the reference voltage and are provided in the comparing means, are controlled. For example, in the case that the demodulating circuit uses the overlap characteristic (namely, calling rate characteristic in overlap area) with a top priority, the reference voltage is set to 44 to 64%, preferably on the order of 47 to 55% with respect to the maximum voltage amplitude of the detection signal. On the other hand, in the case that the demodulating circuit uses the band characteristic (namely, bandwidth characteristic of calling rate) with a top priority, the ratio of the reference voltage with respect to the maximum voltage amplitude of the detection signal is set to be on the order of 0.6 to 0.7.
To control the cutoff frequency of the filter means, when the demodulating circuit employs the digital filter, the digital filter is turned ON/OFF to selectively switch passing and not passing of the signal, so that the cutoff frequency of the overall filter means is changed. Otherwise, the frequency of the clock signal entered to the digital filter is controlled so as to change the cutoff frequency of the digital filter itself. On the other hand, when the demodulating circuit is arranged by the analog type low-pass filter, the cutoff frequency of the low-pass filter is varied by switching the circuit constant of this analog low-pass filter by operating, for instance, a switch and the like.
While the cutoff frequency of the filter means is controlled, when the electric field strength is lower than a preselected electric field strength threshold value, the cutoff frequency is controlled to be decreased, whereas when the electric field is higher than a preselected electric field strength threshold value, the cutoff frequency is increased.
In the case that the ratio between the voltage amplitude of the detecting signal and the reference voltage is controlled and further the cutoff frequency of the filter means is controlled by the control means, while the control condition is saved in the memory means, the operation setting values for controlling the subject are selectively switched by two stages in accordance with the electric field strength information and the control condition information by, for example, employing a threshold value between the low electric field and the field higher than the medium electric field. Alternatively, while plural sets of threshold values for the electric field strength are provided, the operation setting values are selectively switched in multiple stages, so that more precise controls may be carried out in response to the electric field strength. It should be understood that the control condition information may be selectively switched, depending upon a difference in reception environments such as use areas and signal transfer speeds. As a result, even in the overlap area and the multipath area which occur under high electric field and medium electric field and the reception performance of which is deteriorated, the demodulating operation can be carried out under optimum condition. Thus, the overlap characteristic can be improved, and the error rate of the demodulation signal can be maintained to the low rate. Therefore, the reception preformation of the wireless receiving apparatus can be improved.
When the control means is constituted by such a hardware circuit as a logic circuit, more highspeed response can be achieved in this control means. Alternatively, either the control means or the entire demodulating circuit may be arranged by a processor for executing a software processing operation.